FIG. 1a schematically illustrates a representative optical fibre link 2 in a conventional WDM optical communications system. In the illustrated example, the fibre link 2 comprises three optical fiber spans 4 extending between a transmitter node 6 and a receiver node 8, and traverses an optical amplifier 10 and a Reconfigurable Optical Add-Drop Multiplexer (ROADM) 12. As is well known in the art, optical fibre links commonly have multiple spans, and include a variety of optical devices, such as optical amplifiers and ROADMs, for example. Transmitters and receivers are commonly incorporated into network nodes which provide some combination of signal regeneration, electrical switching (such as wavelength switching), and layer-2 (or higher) signal routing functionality. Typically, a bidirectional optical link comprises a pair of parallel fibre links 2 extending between the two end nodes. Normally, these parallel fibre links will be constructed as a “mirror image” of each other, so as to convey optical signals in respective opposite directions. For this reason, only one fibre link is shown in FIG. 1a. 
The transmitter node 6 generally comprises a set of n parallel channel transmitters 14, each of which transmits a respective optical data signal within an optical channel having a predetermined center wavelength. Typically, the number and wavelength spacing of optical channels follows a predetermined spectral grid, such as for example, those published by the International Telecommunications Union (ITU). An optical MUX 16 combines each of the wavelength channels into a WDM signal that is launched through the optical fibre link 2.
The receiver node 8 generally comprises and optical DEMUX 18, which receives a WDM signal through the optical fibre link 2, and routes each of the wavelength channels to respective different channel receivers 20 for detection and data recovery using techniques known in the art.
As is known in the art, a Reconfigurable Optical Add-Drop Multiplexer (ROADM) 12 can be constructed as shown in FIG. 1b. The ROADM 12 generally comprises a 1×m Wavelength-Selective Switch (WSS) 22 which includes a common-IN port 24, a common-OUT port 26 and set of m switch ports 28. Each switch port 28 comprises an input 30 and an output 32. In operation, the WSS 22 is designed to selectively switch any wavelength channel from the common-IN port 24 to the output 32 of any one of the switch ports 28, and to selectively switch any wavelength channel received through the input 30 of any given switch port 28 to either the common-OUT port 26 or to the output 32 of any one of the other switch ports 28. In the ROADM 12 of FIG. 1b, switch port No. 1 is used to support local Add/Drop traffic. Thus, a set of wavelength channels to be dropped is identified, and the WSS 22 controlled to switch these channels of WDM signal received through the common-IN port 24 to the output 32 of switch port No. 1, while routing the other channels to the common-OUT port 26. The dropped channels are then routed to a local demux 34, which supplies each dropped channel to a respective channel receiver 36 for detection and data recovery using methods known in the art. Conversely, channels to be added are generated by respective channel transmitters 38, combined into a WDM signal by optical MUX 40, and supplied to the input 30 of switch port No. 1. The WSS 22 is then controlled to switch these channels from the input 30 of switch port No. 1 to the common-OUT port 26.
For simplicity of illustration, no optical connections are shown in respect of the other switch ports 28 (that is, switch ports 2 . . . m). Typically, at least some of these switch ports 28 would be used to enable optical channel switching to another WSS (i.e. of the same or a different ROADM. Among other things, this functionality can be used to support branching in an optical mesh network.
The maximum number N of wavelength channels in the network is determined by the spectral grid upon which the network was designed. However, the number of active wavelength channels at any given time may be less than this maximum number N of wavelength channels. For the purposes of the present discussion, an active channel is a channel in which an optical signal is present and is being controlled by the steady-state network control systems of the network. Conversely, an inactive channel is a channel for which the corresponding channel transmitter 14,32 is not generating an optical signal, either because it is un-powered or in fact it has not been installed. In a dynamic optical network, the number of active channels typically varies with the data traffic load. Thus, in the example of FIG. 1a, the number of active wavelength channels between the transmitter node 4 and the OADM 12 may be different from the number of active channels between the OADM 12 and the receiver node 8, and the number of active channels in each of these sub-spans may change with time.
The specific number and distribution of active wavelength channels within a given span of the network may be referred to as the “spectral loading” of that portion of the network. Adding or subtracting wavelength channels (by changing an inactive channel to an active state or visa versa, for example) changes this spectral loading.
As is known in the art, dynamic optical networks are controlled, optimized, and reconfigured using a multi-layer hierarchal analog network control system. Such a control system is required because an optical network behaves differently depending on the spectral loading. When the spectral loading of the network changes, other active channels in the network can be perturbed resulting in loss of data. In order to avoid this problem, a multi-step sequential and iterative process is followed in order to transition the network from the previous spectral loading state to the new state. Applicant's co-pending U.S. patent application Ser. No. 11/533,166, filed Sep. 19, 2006 and entitled “Control Of Parameters In A Global Optical Controller”, teaches methods of this type. Thus, for example, in order to add a channel, the method of U.S. patent application Ser. No. 11/533,166 first examines the performance of the existing system to determine if operating conditions in the communications system (e.g. the traffic load and noise margin) are conducive to addition of a new optical channel. If operating conditions are conducive to addition of the new channel, a desired power level of the new channel and then the power level of the incoming channel gradually increased to that desired power level. If operating conditions continue to be conducive to addition of the new channel, a weighting coefficient associated with the new optical channel in a cost function of the optical communication system is gradually increased, so that the steady-state control and optimization functions of the network will fine tune the operation of the new channel. If, at any point during the above process, the performance of any existing channels drops below a predetermined threshold, then the addition of the new channel is aborted. In addition to the complexity involved in implementing methods such as this, there is also the shortcoming that reconfiguration of a large optical network can take a substantial amount of time (On the order of 10 s of minutes).
Techniques enabling reconfiguration of an optical network that overcome limitations of the prior art remain highly desirable.